The carbon allotrope consisting of a one-atom-thick hexagonal lattice is graphene.
Graphene is a single layer of carbon atoms arranged in a flat honeycomb-like structure. Each carbon atom is bonded to three neighboring atoms through sp² bonding, while electrons in the remaining p orbital form a delocalized electronic system. This arrangement gives graphene unusual electrical, thermal, mechanical, and optical properties.
Graphite is made of many graphene layers stacked together, with relatively weak forces between the layers. That distinction explains why graphite can leave marks on paper: its layers slide apart easily. A carbon nanotube can be thought of as a graphene sheet rolled into a cylinder, whereas diamond has a three-dimensional network of sp³-bonded carbon atoms.
The theoretical electronic structure of graphene was studied by physicist P. R. Wallace in 1947. The material became widely known after Andre Geim and Konstantin Novoselov isolated graphene flakes from graphite in 2004 using mechanical exfoliation. Their work earned the 2010 Nobel Prize in Physics.